1. Field of the Invention
The present invention relates to a charging apparatus and a charging current detecting circuit thereof, and more specifically, to a charging apparatus and a charging current detecting circuit that charge a secondary battery by supplying a charging current from a power supply to the secondary battery.
2. Description of the Related Art
Generally, a charging current detecting circuit for detecting a charging current supplied from a power supply to a secondary battery is provided in a charging apparatus for charging the secondary battery.
FIG. 2 shows an example of a charging apparatus 2, such as an AC adapter, comprising such a charging current detecting circuit 1.
The charging apparatus 2 comprises a power supply 3 having a predetermined voltage (24 V in FIG. 2). By supplying the charging current to a secondary battery 4 using the power supply 3, the secondary battery 4 is charged.
An emitter of a controlling transistor 6 for controlling the supply of the charging current is connected to a power voltage side (24 V side) located at the downstream side of the power supply 3, and a collector of the controlling transistor 6 is connected to an anode of the secondary battery 4.
On the other hand, a ground side (0 V side) of the power supply 3 is connected to a cathode of the secondary battery 4.
In addition, a detecting resistor 7 serving as the main element of a charging current detecting circuit 1 is disposed at the ground side located at the downstream side of the power supply 3. The value of the charging current can be detected using the voltage drop caused by the detecting resistor 7.
The upstream side of the detecting resistor 7 is connected to a cathode of a variable battery 5, and the downstream side of the detecting resistor 7 is connected to a non-inversion input terminal of an operational amplifier 9. In addition, an anode of a variable battery 5 is connected to an inversion input terminal of the operational amplifier 9. Further, an output side of the operational amplifier 9 is connected to a base of the controlling transistor 6 via a resistor 8.
The variable battery 5 outputs a charging current setting signal to the operational amplifier 9. In addition, the operational amplifier 9 receives an input of the charging current setting signal and controls electrical conduction of the controlling transistor 6 such that a setting voltage of the charging current setting signal is the same as the voltage drop caused by the detecting resistor 7.
In addition, the operational amplifier 9 is driven by a voltage applied from the power supply 3. Further, when the operational amplifier 9 is driven, the controlling transistor 6 controls the supply of the charging current to the secondary battery 4. (See Japanese Unexamined Patent Application Publication No. 05-137276)
In the charging apparatus 2 as shown in FIG. 2, the detecting resistor 7 is disposed at the ground side (0 V side) of the downstream side of the power supply 3. However, in this case, the operational amplifier 9, which is inexpensive and is operated until an input voltage is almost 0V, can be used.
However, depending on the particular circuit, the detecting resistor 7 may be disposed at the power voltage side (24 V side).
In the operational amplifier 9 of the above cases, the input voltage range (for example, 21 V) is limited to have a value lower than the power voltage (for example, 24 V). As a result, the input voltage (2 V) is insufficient to drive the operational amplifier 9.
On the other hand, if the detecting resistor 7 is disposed at the power voltage side, an operational amplifier 10, such as a rail-to-rail operational amplifier that operates suitably under the power voltage may be used. However, the rail-to-rail operational amplifier is high in cost (FIG. 3)
According to the conventional art, because the detecting resistor 7 is disposed at the ground side (0 V side), the ground of the secondary battery 4 is connected to the ground of the system, as in an electronic apparatus that is connected to the charging apparatus 2, by the detecting resistor 7 without being directly connected to each other.
For this reason, when the system is driven by the power 3 of the charging apparatus 2 without using the secondary battery 4 or when the system is driven by the secondary battery 4 charged by the charging apparatus 2, a loss in voltage is generated due to a voltage drop caused by the detecting resistor 7.